The invention relates to a process for separating components from liquid and gaseous media using nanocomposites and also to a process for producing the nanocomposites.
Molecular or macromolecular components are generally separated from solutions and from the gas phase by absorption processes and/or by chemical bonding of the molecular or macromolecular components to the surface of a solid. These components occasionally also react with the surface. However, these processes are usually not reversible.
To maintain a cost-effective process, the surfaces are generally regenerated. If this cannot be carried out (e.g. in various cases in which activated carbon is used), then the entire adsorbent-adsorbate system has to be disposed of. A problem which occurs in all adsorption processes from gas and liquid phases and is difficult to solve physically is the fact that as the surface area increases (increase in the adsorption capacity) the flow resistance of beds or fixed beds increases by several orders of magnitude as a function of the particle size. This is particularly problematical in fluidized beds. This has a very severe adverse effect on the effectiveness of adsorber beds. A remedy would be to keep very small particles in suspension. However, these have to be filtered off again, and the problem of low liquid throughput is merely shifted from the adsorption step to the filtration step, but is not solved.
It is an object of the invention to find materials and processes which do not have the abovementioned technical drawbacks.
An interesting approach is to employ magnetizeable and/or magnetic particles which are kept in suspension for the adsorption of particular components and then to separate these from the reaction medium (liquid or gas phase to be purified) by application of a magnetic field. An important aspect is the adsorption selectivity of surfaces. This is generally only achieved in a group-specific manner in the case of adsorption media, i.e. a chemical family having similar functionality is generally always adsorbed on a particular surface. Provision of typical adsorbents, e.g. activated carbon or aluminum hydroxide, with selective functions is difficult, since the attachment of such groups to these absorption media is only possible in a few cases in industry. Use is therefore made of less selective methods, e.g. making surfaces hydrophobic or hydrophilic, or attempts are made to influence the acidic or basic character of, for example, aluminum oxides and the loading of the adsorption sites by means of doping.
Magnetic precipitation of magnetic particles is achieved by coating mica platelets with iron oxides and depositing a glass layer which is capable of more or less selectively adsorbing particular biochemical components on these platelets. However, this is a relatively unselective process and is not suitable for the separation of relatively small molecules from liquids and gases. Only macromolecules which have an appropriate surface charge are adsorbed.
Magnetic particles have to have sizes in the nanometer range if they are not to display permanent magnetism. Permanent magnetism after a magnetic field is switched off would lead to aggregation by mutual interaction between the particles. The particle size is therefore preferably  less than 20 nm in order to achieve superparamagnetic behavior (single domain structures).
The object of the invention is achieved by producing superparamagnetic nanoparticles as are described in the German Patent Application 19614136. To prevent irreversible aggregation, these particles are coated with functional groups, e.g. amines, amino groups or carboxylate groups. Since amino groups are good complexing agents for transition metals, it is in principle possible to employ such particles for binding transition metal ions in aqueous solutions.
However, separation by means of a magnetic field is incomplete because of the small size of the particles and the associated Brownian motion, so that relatively large amounts of the particles are always entrained in a flowing medium. The object was able to be achieved by dispersing the nanoparticles in a liquid or dissolved matrix phase and producing relatively large particles having desired diameters from this matrix phase. The diameter of these particles is in the range from 0.1 to 1,000 xcexcm, but preferably in the range from 1 to 500 xcexcm and particularly preferably in the range from 50 to 300 xcexcm.
The particles can be produced by introducing the liquid matrix phase into an immiscible solvent and producing an emulsion having the correct size range by means of a mechanical mixing process (e.g. Ultraturrax stirrer). This method allows the abovementioned particle sizes to be produced. During the introduction of mechanical energy, which can be supplemented by ultrasound, a solidification reaction according to customary principles takes place. This can be a polymerization reaction, a precipitation reaction, an addition reaction or a polycondensation reaction. The preferred type of reaction depends on the matrix system which can be produced from alkoxides (sol-gel process) but also from organic monomers, oligomers or dissolved phases. In addition, functional groups can be incorporated in this matrix, e.g. by use of functional silanes or functional double-bond-molecules. This matrix functionalization, which also functionalizes the surfaces of the nanocomposite particles, can be used to generate reaction selectivity (e.g. complex formation with heavy metals). Another variant of the process is subsequent surface modification, e.g. by silanization of sol-gel nanocomposite particles.
Such nanocomposite particles make it possible to produce suspensions in aqueous or organic solvents. However, they are also suitable for fluidized-bed processes in the gas phase. Appropriate choice of the functional groups on the surface makes it possible to separate off both ionic components and components which can be bound by complex formation as well as biological and biochemical components when appropriate functional groups (antibodies, antigens, proteins or the like) are bound to the surface. The boundary conditions are selected so that the components are bound, for example, in pH ranges in which the association constant of complex formation is very low. After the particles have been laden with the components to be removed, which can be accelerated by stirring, a magnetic field is switched on so as to attach the particles to the wall or to a device which is introduced into the suspension. This device is then removed together with the collected particles and the bound components are eluted in a regeneration step, after which the nanocomposite particles are once again separated off magnetically and are then available for a further purification procedure. Such processes can also be operated continuously when suitable plants are used. Gas-phase purification in a fluidized bed is carried out analogously.
The novel process described can thus be employed for a large number of purification processes from solutions.